Typically, in the construction of homes it is important to protect roofs from leaks due to ice and rain. Traditionally, felt paper was secured to wooden roofs underneath shingles. The felt paper would absorb ice or water that penetrated the shingles, preventing it from reaching the underlying wood. Nailing the felt paper to the roof, however, caused spaces around the nail through which water could seep. The water could follow the nail into the wood, causing leaks in the home. To solve this problem, water shields began to include an adhesive backing to fasten the shield to the wood, instead of using nails. The adhesive backing includes a peel-able strip which, when removed, exposes the adhesive layer for affixing the water shield to the unprotected wooden roof. The top of these water shields were made of a rubberized asphalt material, which created a gasket effect on the shaft of the nail driven through it. These water shields were successful in preventing many types of leaks.
In colder climates, however, ice dams can form and allow water to penetrate or flow under the water shield. For example, an ice dam can prevent melt-water from flowing downward off the roof, which can result in the water seeping into the house above the ice and water shield coverage area. Ice dams occur when snow accumulates on the roof of a house with inadequate insulation. Heat conducted through the insufficiently insulated roof, and warm air from the space below, warms the roof and melts the snow on areas of the roof that are above living spaces. It does not, however, melt the snow over cold areas, such as roof overhangs. In these situations, melt-water from the heated areas of the roof flows down the roof, under the blanket of snow, onto the overhang and into the gutter, where colder conditions permit it to freeze. Eventually, ice accumulates along the overhang and in the gutter. Snow that melts later cannot drain properly, backs up on the roof and can result in damaged ceilings, walls, roof structure, and insulation. To avoid this many building codes require a water shield covering the roof two feet into the living space.